This invention is directed to dendrimer-based networks containing lyophilic organosilicon and hydrophilic polyamidoamine (PAMAM) and/or polypropyleneimine (PPI) nanoscopic domains, and more particularly to silicon-containing dendrimer-based elastomers prepared from radially layered copoly(amidoamine-organosilicon) (PAMAMOS) and/or copoly(propyleneimine-organosilicon) (PPIOS) dendrimer precursors.
During the last ten years, dendritic polymers have become one of the fastest growing areas of research interest in polymer science. There are at least two main reasons that seem responsible for this unusual amount of interest in such new polymers. First, it has been clearly realized that dendritic polymers represent a fourth main class of macromolecular architecture, which can be described as tree-like macromolecules consisting of unique branch-upon-branch-upon-branch structural organizations. Second, and based on the fundamental importance of molecular architecture for imparting macroscopic properties and behavior to polymeric materials, it is expected that these new polymers would exhibit properties that are not found in any other class of conventional polymer architecture, including linear, randomly branched, and crosslinked macromolecules.
Among the dendritic polymers, particular attention has been focused on dendrimers representing globular macromolecules having branch junctures in every repeat unit, as well as unusually precise molecular shapes, sizes, and functionality. Depending on their chemical composition, these macromolecules assume shapes from almost spherical to ellipsoidal. Such macromolecules span the lower domain of the nanoscopic size region, i.e., from about 1 to about 15 nanometer (nm) in diameter with regular increments of about 0.7 to 1.3 nm per generation. Typically, the macromolecules contain from only few to several thousand inert or reactive surface groups. Reference may be had to Table 1 below, for an example of one representative type of such a macromolecule.
TABLE 1 ______________________________________ Molecular characteristics of ethylenediamine (EDA) core polyamidoamine (PAMAM) dendrimers Number of Hydrodynamic Radius, .ANG. Generation Surface Groups MW SEC DSV SANS ______________________________________ 0 4 517 7.6 -- -- 1 8 1,430 10.8 10.1 -- 2 16 3,256 14.3 14.4 -- 3 32 6,909 17.8 17.5 17.8 4 64 14,215 22.4 25.0 26.4 5 128 28,826 27.2 32.9 33.5 6 256 58,048 33.7 -- 43.3 7 512 116,493 40.5 -- 50.6 8 1024 233,383 48.5 -- -- 9 2048 467,162 57.0 -- 65.1 10 4096 934,720 67.5 -- -- ______________________________________
The Hydrodynamic Radius shown in Table 1 was determined at 25.degree. C., pH of 2.7, using 0.1 molar citric acid in water. Values are reported as obtained by using Size Exclusion Chromatography (SEC) relative to linear polyethylene oxide standards; Dilute Solution Viscometry (DSV), and Small Angle Neutron Scattering (SANS).
In addition to these characteristics, dendrimers can be obtained with almost perfect monodispersity, having weight average molecular weight/number average molecular weight (M.sub.w /M.sub.n) coefficients routinely below 1.02, and ranging in molecular weights from only several thousand to as high as a million or more. Hence, because of their unprecedented structural regularity and high functionality, dendrimers represent precisely defined nanoscopic building blocks available for preparation of more complex supermolecular nanoconstructions that have not been previously attainable by other synthetic means.
The first well defined, symmetrical, dendrimer family were the polyamidoamine (PAMAM) dendrimers of the general structure depicted in our FIGS. 2 and 3. In particular, FIG. 2 shows the structure of an ethylenediamine (EDA) core, Generation 1 dendrimer, and FIG. 3 shows the structure of the PAMAM repeating unit. These dendrimers are commercial products sold under the trademark STARBURST.RTM. by Dendritech, Incorporated, Midland, Mich., U.S.A.
Silicon containing dendritic polymers have gained increasing attention only recently. We are aware of reports on three different main silicon containing dendrimer families. The most widely utilized family has been the carbosilane dendrimers which have been used for various surface modifications. Their preparation was originally described in J. Chem. Soc., Chem. Commun., Pages 1400-1401, (1992). Polysiloxane-based dendrimers have also been reported, but the synthetic methodology for their preparation has not been found too practical. See for example, Dokl. Akad. Nauk. SSSR, 309, Pages 376-380, (1989), and Macromolecules, Volume 24, Number 12, Pages 3469-3474, (1991). Most recently, polysilane-based dendrimers have been reported, although their syntheses have been successful only to very low generations. See for example, Angew. Chem. Int. Ed. Engl., 34, No. 1, Pages 98-99, (1995); J. Am. Chem. Soc., Volume 117, No. 14, Pages 4195-4196, (1995); and Chemistry Letters, Pages 293-294, (1995).
In comparison, the uniqueness of our invention resides in the preparation of networks first from copolydendrimers having a covalently bonded hydrophilic PAMAM or PPI interior and an oleophilic hydrophobic organosilicon radially concentric outer layer. While some work has been done by others in preparing hydrophobic dendrimers with hydrophilic interiors, the prior work has not included organosilicon outer layers. See for example, Polymer Preprints, Volume 37, Number 2, Page 247, (1996).
Thus, our PAMAM-organosilicon layered dendrimers (PAMAMOS), in which a hydrophilic PAMAM comprises the interior, while organosilicon generations are built on top of and around it, represent the first dendrimers of this kind which can be obtained with both inert and reactive functional groups, on the outer surface of the dendrimer, including for example, groups such as (CH.sub.3).sub.3 Si--, (CH.sub.3 O).sub.a (CH.sub.3).sub.3-a Si--, and (CH.sub.2 .dbd.CH).sub.a (CH.sub.3).sub.3-a Si--, where a is 1, 2, or 3. Reference may be had to our prior copending application.
Accordingly, the number of reactive functional groups on the PAMAMOS or PPIOS dendrimer surface can be varied, depending upon (i) the functionality of the starting PAMAM or PPI dendrimer reactant; (ii) the completeness of its modification by the organosilicon reagent; and (iii) the functionality (Z) of the organosilicon reagent. In general, the functionality of PAMAM or PPI dendrimers is dictated by the functionality of the initiator core reagent used in their synthesis, and by generation. For example,,when ethylene diamine (EDA) is used as initiator core reagent, the functionality of the amine terminated PAMAM dendrimers ranges from 4 at generation 0 to 4096 at generation 10, doubling from every generation to the next, as shown in Table 1 above. For complete substitution, the number of functional groups of organosilicon derivatized PAMAM dendrimers (PAMAMOS) is determined by the relationship Z=Z.sub.PANAM N.sub.b.sup.G, where Z.sub.PANAM is the number of functional groups of the starting PAMAM dendrimer; N.sub.b is the branching functionality of the organosilicon modifier used in the preparation of the PAMAMOS copolydendrimer which may be 2 or 3; and G is the number of organosilicon layers, i.e., generations around the PAMAM interior.
Up until our present invention, we are not aware of any reported work on the utilization of silicon-containing dendrimers for preparation of more complex nanoscopic products. Yet, the concept of dendrimer-based networks is gaining increasing attention. Such networks can result from establishing a three-dimensional covalent connectivity between individual dendrimers. In principle, such connectivity can be established
(i) between the surfaces of two adjacent dendrimers, (ii) between one dendrimer surface and another dendrimer interior, and PA1 (iii) between interiors of two neighboring dendrimers. PA1 (1) catalyzed addition reactions such as hydrosilation or thiol addition, in the case of .tbd.SiCH=CH.sub.2, .tbd.Si--CH.sub.2 --CH.dbd.CH.sub.2, .tbd.Si--R--SH, or .tbd.SiH surface functionalized dendrimers; PA1 (2) self-catalyzed reactions such as hydrolysis with moisture or water, in the case of .tbd.SiCl and .tbd.Si--OR surface functionalized dendrimers; PA1 (3) non-catalyzed addition reactions such as Michael addition; and PA1 (4) condensation reactions.
Accordingly, as can be seen in FIG. 1, our invention is directed to the first of these three possible scenarios. As such, connectivity is achieved either (i.a.) by reacting two types of dendrimers having different but mutually reactive functional groups, or (i.b.) by reacting a particular dendrimer with an appropriate difunctional, trifunctional, or polyfunctional connector(s).
Both of these approaches have been demonstrated but only with pure PAMAM dendrimers. Thus, by the first approach, amine surface PAMAM dendrimers have been reacted in a classical amidation reaction with carbomethoxy surface PAMAMs, to produce higher dendrimer agglomerates. See for example, U.S. Pat. No. 4,568,737 (February 1986), U.S. Pat. No. 4,713,975 (December 1987), and U.S. Pat. No. 4,737,550 (April 1988). By using the second approach, carbomethoxy surface PAMAMs have been reacted with ethylene diamine, and amine surface PAMAM dendrimers have been treated with K.sub.2 PtCl.sub.4, to prepare ordered dendritic multilayers. See for example, Polymer Journal, Volume 17, No. 1, Pages 117-132, (1985); and J. Am. Chem. Soc., Volume 116, No. 19, Pages 8855-8856, (1994); respectively. It should be noted, however, that the products obtained were not elastomers, but rather amorphous solids at room temperature.
In addition to this, organosilicon networks described as being "somewhere in between inorganic glasses and organic elastomers", were also prepared. However, the precursors used were not dendrimers, but small molecular weight multi-functional branched compounds containing 12 alkoxysilane groups per molecule, emanating from either a single silicon atom, a linear disiloxane segment, or a ring system. See for example, M. J. Michalczyk and K. G. Sharp, 29th Organosilicon Symposium, Evanston, Ill. (Mar. 22-23, 1996).